1. Field of the Invention
The present invention relates to a transmission electron microscope and, more particularly, to an optical system for correcting spherical aberration due to the objective lens.
2. Description of the Related Art
In transmission electron spectroscopy (TEM), it is desired to minimize the value of resolution d. For example, where the atomic arrangement of crystals or the like is directly visualized by TEM, the resolution d is required to be as high as about 0.1 nm.
The resolution d of a transmission electron microscope (TEM) is determined by the spherical aberration coefficient Cs of the objective lens as given by                               xe2x80x83                ⁢                  d          =                      0.65            ⁢                          xe2x80x83                        ⁢                          Cs                              1                4                                      ⁢                          λ                              3                4                                                                        (        1        )            
where xcex is the wavelength of electrons used. Therefore, it can be seen from Eq. (1) that the resolution d of the TEM can be reduced either by shortening the wavelength of the electrons or by reducing the spherical aberration coefficient Cs.
However, the polepieces of today""s TEMs are optimized almost to the machining limit. Consequently, it is very difficult to reduce the spherical aberration coefficient Cs further.
On the other hand, it is relatively easy to increase the accelerating voltage. As mentioned above, the wavelength xcex of the electrons can be shortened by increasing the accelerating voltage. Thus, the resolution d can be reduced. In practice, a TEM with an accelerating voltage of 1000 kV can provide a resolution of about 0.1 nm. However, increasing the accelerating voltage increases the size of the instrument.
Accordingly, relatively small-sized TEMs with accelerating voltages on the order of 200 kV are required to exhibit resolutions of approximately 0.1 nm. In this class of TEM, xcex=0.00251 nm and Cs=0.5 mm. Therefore, the present situation is that the resolution d is limited to about 0.19 nm.
As described thus far, the polepieces of TEMs have been optimized nearly to the machining limit. Accordingly, where one attempts to obtain a resolution of about 0.1 nm, the accelerating voltage must be increased. Therefore, the present situation requires that the instrument is made bulky.
If conventional polepieces are used, and if spherical aberration can be corrected even at an accelerating voltage of about 200 kV, the resolution can be improved. However, a TEM using a cylindrically symmetric round magnetic lens cannot correct spherical aberration. The round lens is so designed that the geometrical arrangement of lens properties is not affected by rotation of the lens about the optical axis. This is referred to as xe2x80x9caxial symmetryxe2x80x9d.
That is, a round magnetic lens is normally used as an objective lens for a TEM. Unfortunately, the round magnetic lens cannot form a concave lens. Therefore, spherical aberration cannot be corrected by any combination of round magnetic lenses.
Accordingly, introduction of a magnetic field produced by multipole elements has been proposed to correct spherical aberrations (e.g., O. Scherzer, Optik 2 (1947), p. 114). This makes use of the fact that where hexapole elements, for example, are used as multipole elements, an aberration term of the intrinsic trajectory given to the electron by the hexapole elements appears as an effect that corrects the spherical aberration coefficient Cs of the objective lens. In particular, a combination of two transfer doublet lenses and two hexapole elements has been proposed to correct spherical aberrations (M. Haider, G. Braushausen, and E. Schwan, Optik 99 (1995), p. 167). One example of a post-specimen imaging optical system is shown in FIG. 10.
However, the optical system consisting of a combination of two transfer doublet lenses and two hexapole elements as shown in FIG. 10 is complex in structure. In addition, the microscope column is elongated, making the instrument large. The reason why the instrument is made large is summarized as follows. One transfer doublet lens is made up of two transfer lenses. Let fT be the focal distance of each doublet lens (i.e., the focal distance of the transfer lens). To accommodate this one doublet lens made up of two transfer lenses, a space as long as 4fT is necessary in the microscope column. Therefore, assuming that the two doublet lenses have a focal length of fT, a space having a length of 8fT is necessary to accommodate these two doublet lenses. Since the focal lengths fT of the doublet lenses are approximately 30 to 50 mm, a length of 24 cm is needed in the microscope column only if two doublet lenses are accommodated, provided that fT is 30 mm.
Accordingly, it is an object of the present invention to provide a transmission electron microscope which is capable of correcting spherical aberration even if an accelerating voltage of about 200 kV is used and which has a microscope column that can be made shorter than that of the prior art instrument described-above.
The object described above is achieved by a transmission electron microscope which is built in accordance with the present invention and comprises: an illumination system including two illumination lenses interlocked in such a way as to form an image of a light source at a fixed first crossover point that is in immediate vicinity to a specimen upstream thereof; an objective lens; a first multipole element for producing a three-fold symmetrical field, the first multipole element being placed at a position where the objective lens forms the first crossover point as a second crossover point that is in immediate vicinity to the specimen downstream thereof; a second multipole element for producing a three-fold symmetrical field, the second multipole element being located downstream of the first multipole element; and a doublet lens located between the first and second multipole elements to project the second crossover point as a third crossover point onto the position of the second multipole element.
The present invention also provides a transmission electron microscope comprising: an objective lens for forming an electron diffraction image of a specimen; an illumination optical system for fixing the electron diffraction image at a position; a first multipole element for producing a three-fold symmetrical field, the first multipole element being located at the position where the electron diffraction image is formed; a second multipole element for forming a three-fold symmetrical field, the second multipole element being located in a position conjugate with the first multipole element; and a doublet lens located between the first and second multipole elements to project an electron diffraction image formed at the position of the first multipole element onto the position of the second multipole element.
More specifically, the present invention provides a transmission electron microscope comprising:
an objective lens for forming an electron diffraction image of a specimen; an illumination optical system for fixing the electron diffraction image at a position independent of a current density on a specimen; a first multipole element for producing a three-fold symmetrical magnetic field having a center where the electron diffraction image is formed; a second multipole element for producing a three-fold symmetrical magnetic field having a center located in a position conjugate with the first multipole element; and a doublet lens located between the first and second multipole elements to shift the electron diffraction image formed in the center of the first multipole element to the center of the second multipole element.
The present invention also provides a transmission electron microscope comprising: an illumination lens and an auxiliary illumination lens that are interlocked to form an electron diffraction image at a back focal plane of an objective lens at all times independent of a current density on a specimen; the objective lens forming a hexapole field and having a center brought into coincidence with the back focal plane of the objective lens; a multipole element for producing a three-fold symmetrical magnetic field located in a position conjugate with the back focal plane that is the center of the hexapole field; and a doublet lens located between the hexapole field and the multipole element to shift the electron diffraction image formed at the back focal plane to the center of the magnetic field produced by the multipole element, the back focal plane being in the center of the magnetic field of the hexapole field.
Other objects and features of the invention will appear in the course of the description thereof, which follows.